A typical prior art a disk drive system 10 using longitudinal recording is illustrated in FIG. 1. In operation the magnetic transducer (head) 20 is supported by a suspension (not shown) as it flies above the rotating disk 16. The magnetic transducer 20, usually called a “head” or “slider,” is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The magnetic transducer 20 is positioned over points at varying radial distances from the center of the disk 16 to read and write circular tracks (not shown). The disk 16 is attached to a spindle (not shown) driven by a spindle motor (not shown) to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded.
Reference is made to FIG. 2 to illustrate the thin film layers in a particular embodiment of a magnetic film disk 16. The substrate 26 is glass. An initial thin film deposited on the substrate will be called a pre-seed layer 31. The use of pre-seed layers is relatively recent practice. The pre-seed layer is an amorphous or nanocrystalline thin film that is deposited on the substrate prior to the crystalline seed layer. The behavior of the pre-seed layer is as an amorphous material, but it is known in the art that amorphous behavior can result from materials which have a nanocrystalline structure. The preseed layer helps to improve media magnetic properties and recording performance and provide improved mechanical properties for the disk. The seed layer 32 is deposited onto the pre-seed layer. Seed layers are commonly used with nonmetallic substrate materials such as glass. Typically the seed layer is the first crystalline film deposited in the structure and is followed by one or more crystalline underlayers. Typically both the pre-seed layer and seed layer are relatively thin layers. Materials proposed for use as seed layers include chromium, titanium, tantalum, Ni3P, MgO, carbon, tungsten, AlN, FeAl, NiAl and RuAl. In U.S. Pat. No. 5,789,056 to Bian, et al., the use of a CrTi seed layer is described. The use of two seed layers is also known. The films also include one or more underlayers 33 which are commonly chromium or chromium alloy such as CrV and CrTi. The magnetic layer stack 34 includes at least one ferromagnetic layer based on various alloys of cobalt. For example, a commonly used alloy is CoPtCr. Additional elements such as tantalum and boron are often used in the magnetic alloy. A protective overcoat layer 35 is used to improve wearability and corrosion resistance.
The preferred orientation (PO) of the various crystalline materials forming the layers on the disk, as discussed herein, is not necessarily an exclusive orientation which may be found in the material, but is merely the most prominent orientation. When the Cr alloy underlayer is sputter deposited at a sufficiently elevated temperature on a NiP-coated AlMg substrate a [200] PO is usually formed. This PO promotes the epitaxial growth of [11-20] PO of the hexagonal close-packed (hcp) cobalt (Co) alloy, and thereby improves the magnetic performance of the disk. The [11-20] PO refers to a film of hexagonal structure whose (11-20) planes are predominantly parallel to the surface of the film. Likewise the [10-10] PO refers to a film of hexagonal structure whose (10-10) planes are predominantly parallel to the surface of the film. The [10-10] PO can be epitaxially grown on an appropriate underlayer with a PO of [112].
One technique used in the prior art to improve magnetic recording performance on thin film disks is circumferential polishing to create a pattern of fine “scratches” (circumferential texture) which are generally oriented along tracks (concentric circles) on the disk surface. The scale of the texture of commercial thin film disks is microscopic with a peak-to-valley of less than 5 nm typically. A 5 nm texture appears mirror-like to the unaided eye. Special polishing equipment is necessary to achieve circumferential texture this fine such as is described in Jones, et al., U.S. Pat. No. 5,490,809. The topography of the surface on which a thin film is deposited can have a significant effect on the way the film nucleates and grows and also upon its characteristics. Circumferential texture on magnetic disks has been commonly used to influence the in-plane magnetic anisotropy for a wide range of magnetic alloys. For longitudinal recording it is sometimes useful to have a higher coercivity (Hc) in the circumferential direction than in the radial direction. The ratio of the circumferential Hc to the radial Hc is called the Hc orientation ratio (HcOR). The ratio of the circumferential Mrt to the radial Mrt is called the Mrt orientation ratio (MrtOR). Current disks typically use hexagonal close packed (hcp) cobalt alloys and most (but not all) circumferentially textured disks have an Hc and Mrt OR>1.
U.S. Pat. No. 6,567,236 to Doerner, et al., describes a preferred embodiment of a layer structure as: a pre-seed layer preferably of CrTi, a seed layer preferably of RuAl, an underlayer preferably of CrTi, a bottom ferromagnetic layer preferably of CoCr, an antiferromagnetic coupling/spacer layer preferably of Ru; and a top ferromagnetic structure including: a thin first sublayer of material preferably of CoCr, CoCrB or CoPtCrB, and a thicker second sublayer of material preferably of CoPtCrB with a lower moment than the first sublayer.
A thin film disk is described in U.S. Pat. No. 6,596,409 to Bian, et al. that includes a pre-seed layer of amorphous or nanocrystalline structure which may be CrTa or AlTi or AlTa, and that is deposited upon a disk substrate. The pre-seed layer is followed by the RuAl seed layer, a Cr alloy underlayer, an onset layer composed essentially of CoCr and a magnetic layer. The onset layer has an optimal concentration of 28-33 at. % Cr and an optimal thickness of 0.5 to 2.5 nm. It increases coercivity and improves the Signal-to-Noise Ratio (SNR) of the disk. CrTi or CrMo alloys are preferred to form the underlayer.
U.S. Pat. No. 6,287,429 to Moroishi, et al. describes a magnetic recording medium in which an underlayer (called an “intermediate layer” in the text) is formed on the substrate surface, and a magnetic layer formed on the underlayer. The underlayer is composed of chromium (Cr), molybdenum (Mo) and zirconium (Zr) between 0.1 atomic percent (at. %) and 30 atomic percent which is said to improve the coercivity with Zr between 0.1 atomic percent and 4 at. % being preferred. The underlayer can include first and second sublayers. The first sublayer is composed of chromium and is formed on the substrate surface. The second sublayer comprises chromium and molybdenum and is formed on the first layer. An embodiment with an underlayer of CrMoZr and a magnetic layer composed of CoPtMo is described.
Y. Cheng, et al. in U.S. Pat. No. 6,183,860 describe magnetic recording media having a CrMo underlayer preferably with the Mo crystals being at least about 140 ANG. in the film growth direction for the (002) crystal plane preferably with a magnetic layer such as CoCrTaPtNi. Mo in the range between about 7 and 16 at. % in the CrMo alloy, more preferably between about 9 and 11 at. %, will provide the preferred (002) crystal orientation.